In many types of implanted medical device (for example, orthopedic implants, pedicle screws, dental implants, spinal implants and sensors) it is desirable to have a strong interaction between the surface of the device and the surrounding tissues (most commonly bone) for the purpose of load and stress transmission. Such devices are used to stabilize fractures, strengthen weak bones and anchor prostheses.
The surfaces of such devices (hereafter referred to generally as “implants”) have been shown to osseointegrate when surrounded by bone. Osseointegration (the formation of a structural connection between the implant and the surrounding living bone) occurs following implant placement as a result of new bone formation or remodelling of the existing bone which is in direct contact with the implant's surface. Bone may form directly onto the implant surface or there may be a very thin interposed protein layer. Such osseointegration has been demonstrated in many studies histologically, radiographically and with pull out, removal torque, resonance frequency analysis and other mechanical tests.
Implants are typically pure metals, alloys or ceramic devices. Titanium, zirconia, hafnium, tantalum, stainless steel and cobalt chromium are commonly used materials. It is well understood that the surface topography (roughness, surface characterization whether random or repeated) of the implant may influence the rate and quality of bone formation at the implant-tissue interface. In general, it is considered that implants which have been roughened on the nanometer and micrometer scale can increase the rate and quality of bone formation. The consequent reduction in the time taken for healing and osseointegration is highly desirable, enabling early loading and reduced treatment times. In addition the strength and stiffness of the implant-bone interface can be greater with surfaces having certain topographies.
There are a number of well documented methods for the alteration of the surface topography or roughness of implants. These may include particle blasting (grit, sand and other abrasive particles), acid etching, plasma spraying, anodizing, micro-arc oxidation or a combination of these. This may result in a single level of roughness or multiple modulated levels of roughness ranging from a scale of 1 nm to 100 μm. Topography and textures of these types are well known from commercial products and for example from EP 0 388 576.
The surface modification processes described above can also alter the chemistry of the surface. Typically metals form surface oxides on exposure to air and water. Such exposure may occur during production or surgical placement or handling. A reaction with water can occur on the implant surface wherein hydroxyl groups form (Boehm H. P., 1971 Acidic and basic properties of hydroxylated metal oxide surfaces, Discussions of the Faraday Society, 52, 264-275). Chemically, the surface of the implant may be the metal itself, an oxide of the metal, or a hydroxylated surface, for example titanium, titanium oxide or titanium hydroxyl. Carbon and other impurities may be present on the implant surface as a result of the production, storage or handling procedures.
It is highly desirable that, when an implant is placed into the tissues or bone, it is thoroughly wetted with the body's natural tissue fluids. Tissue fluids contain nutrients, electrolytes, proteins, growth factors and other substances essential in the healing and bone formation process. Implants may also be pre-treated with liquids or gels, growth factors for example during production or prior to treatment. Any liquid, gel or solution contacting an implant should thoroughly wet the surface and penetrate any topographical features.
It has been shown that there is a correlation between biocompatibility, bioadhesion and surface tension or contact angle on a substrate or implant surface (Baier, 1972, The role of surface energy in thrombogenesis, Bull. N.Y. Acad. Med. 48, 257-272). One of the major problems with implants having roughened surfaces is the potential hydrophobicity or inability of the surface to wet adequately when liquids are applied to it. This may be due to contamination of the surface with organic or hydrophobic material or to the geometry of the surface preventing penetration of fluid due to surface tension. Wetting, hydrophilicity and hydrophobicity of surfaces measured as the contact angle can readily be deduced using a goniometer or Wihelmy plate.
It is essential that tissue fluids or applied liquids penetrate the topography of a surface completely to ensure that nutrients, proteins and growth factors can maintain cell metabolism, healing and bone formation. However the nature of the topography or texture of the surface is important. Increasing the roughness of a surface may cause air to be trapped under a liquid layer preventing wetting. In addition, the aspect ratio (height or depth of troughs or porosities in relation to their width or circumference) of the topography is critical as this may cause bridging and bridge formation with a failure of a fluid to penetrate such features.
It is therefore an object of the present invention to provide a method of treating an implant whereby the hydrophilicity or wetting of an implant surface may be increased to increase the penetration of liquids onto the surface. Alternatively, or in addition, specific biomolecules could be attracted to the surface. These objects may be achieved over part or the entirety of the implant's surface.